During recent years, maritime activities both commercial and recreational have increased dramatically. As a consequence, visibility of marine articles is more important as U.S. bodies of water become crowded. The current means of increasing visibility of marine articles, such as, for example, buoy markers and boat indicators, include phosphorescent or fluorescent materials and brightly colored paints. However, these current means suffer from serious visibility and durability challenges.
Generally, the afterglow time of a fluorescent substance is short, i.e., the light emitted from the fluorescent substance decays immediately after removal from the source of excitation. Unlike such a fluorescent substance, some substances emit light after having absorbed ultraviolet radiation, and afterglow thereof that can be visually observed for a considerable time (ranging from several tens of minutes to several hours) after the source of stimulus is cut off. Such substances are called phosphorescent phosphors.
Phosphorescent phosphors are known. Sulfide phosphorescent phosphors are also known and include for example CaS:Bi (which emits light of violet blue), CaStS:Bi (which emits light of blue), ZnS:Cu (which emits light of green) and ZnCdS:Cu (which emits light of green) and ZnCdS:Cu (which emits light of yellow or orange). However, many of these sulfide phosphorescent phosphors are chemically unstable and show degraded light resistance, i.e., they suffer from problems that must be solved for practical use.
The most extensively used phosphorescent phosphor among such sulfide phosphorescent phosphors is zinc sulfide phosphorescent phosphor (ZnS:Cu). However, zinc sulfide phosphorescent phosphor is decomposed as the result of irradiation by ultraviolet radiation in the presence of moisture and thus, over time, blackens or reduces the phosphor's luminance. Therefore, it is difficult to use this phosphorescent phosphor in fields where it is placed outdoors and exposed to a direct sunlight, that is, application is limited to luminous clocks/watches or clocks/watches and instrument dials, evacuation guiding signs or indoor night-time display.
Even when zinc sulfide phosphorescent phosphor is used for a luminous clock, the afterglow allows the time to be visually recognized lasts only from 30 minutes to 2 hours. Therefore, the phosphorescent phosphor must be doped with a radioactive substance and a self-luminous paint to keep emitting light by absorbing the energy of radiation from the radioactive substance.
The use of brightly-colored paints, including fluorescent paints, on marine floats do not solve the visibility problem in darkness. Brightly-colored and fluorescent paints can only be seen if light is present to be reflected thereby. Also, these paints become eroded or worn from the relatively soft and pliable surface of microporous resinous or foam marine floats, during use, so that any advantage which justifies the expense of applying such a paint is soon lost or severely diminished.
Thus, a need exists to develop phosphorescent marine products that provide long lasting afterglow characteristics, are chemically stable and physically durable in an outdoor marine environment.